The third release candidate of the long anticipated Firefox 3.5 is now available. Its probably a bug fix release as the release notes does not work and/or have not been made. Clicking the Release Notes link under download links you to Mozilla’s 404 Error Page. You can use the Firefox 3.5 Release Notes for now.
To update, you can use the Check for Updates in the Help menu (if you are on an administrator user account or running Firefox under admin) or you can download the latest version from the Firefox Pre-Release Download Site.
Firefox Pre-Release Download Site (Mozilla)